Yeah, Jazz is heavy on theory too. Especially for improvisation and jamming, you really really need to know what you are doing and which tone will match what the group is doing at the same time.

But to play folk or rock music for example, you do not need that much of it. If you can read music sheet, you can reproduce the song. You can kinda create similar own going by intuition. But, creating the stuff Bach was creating absolutely does require tons of knowledge.